Forty-three (86 per cent) of 50 patients with congestive heart failure, 32 of whom had been refractory to previous diuretic therapy, were completely controlled with hydrochlorothiazide. The latter drug was effective in 24 instances in which chlorothiazide had failed. The incidence of side effects was low and significant electrolyte disturbances did not occur in dosages below 400 mg. daily. A dosage of 200 to 300 mg. hydrochlorothiazide daily is recommended for the treatment of severe heart failure. Thereafter, a dose of 25 to 100 mg. per day will usually maintain the compensated state. The use of hydrochlorothiazide represents a significant advance in the therapeutic armamentarium for congestive heart failure.